The present invention relates to an improved method for removal of nitrates (denitrification) from water that contains nitrates as pollutants.
The nitrate content in underground and surface water is continuously increasing, mainly due to the synthetic or natural fertilizers used in intensive-cultivation agricultural areas.
Ingestion of nitrates by children (infants and young children) is a severe health hazard due to the reduction of nitrates to nitrites and to the subsequent oxidation of hemoglobin to metahemoglobin, which is a stable form and is no longer able to transfer oxygen to the tissues (metahemoglobinemia).
In adults, metahemoglobin is rapidly converted to the oxidized form, which carries oxygen in the blood; accordingly, the World Health Organization believes that a moderate content of nitrites in potable water is not a health risk for adults.
The maximum limit set for nitrites by European Community standards is 50 mg/l; the recommended limit is 25 mg/l and 10 ml/l for children.
The most widely used methods for removing nitrates from water are biological or are based on ion exchange or on the use of reverse-osmosis membranes.
In the case of ion exchange and reverse osmosis, the problem of removing nitrates is transferred elsewhere, since the nitrate-rich water that these processes generate must in turn be disposed.
Biological denitrification uses bacteria which, in an anoxic environment, use organic carbon for growth (heterotrophic processes) or inorganic carbon (autotrophic processes).
In heterotrophic processes, the oxygen that is present in the nitrate molecule is used to oxidize the organic substance (generally sugar) and the nitrates are gradually reduced to elementary nitrogen.
The considerable production of biomass and sludge to be disposed and the frequent countercurrent washing in order to minimize the formation of nitrites are the main limitations of heterotrophic processes.
Moreover, these processes, like autotrophic ones, are characterized by very slow rates.
In autotrophic processes in which the source of inorganic carbon is generally constituted by dissolved carbon dioxide or by the bicarbonates themselves that are present in the water, hydrogen is mostly used as reducing agent.
Only a few microorganisms are capable of surviving and growing in a hydrogen environment: this determines a biological selection of the usable bacteria. Schematically, in autotrophic processes the water is pumped into a pressurized tank, where it is saturated with hydrogen; before entering the reactor, the water receives the addition of carbon dioxide in order to compensate for the loss of H+ ions caused by the denitrification reactions.
The bacterial biomass adheres to a filtration bed with high surface area.
The denitrified water then enters an aeration tank, which reoxygenates the water and strips away the residual oxygen; it then passes over gravity filters which retain any biomass and pollutants that can be filtered out. The last step of the process consists of disinfection with ultraviolet rays in order to inactivate any residual microorganisms.
The autotrophic process does not produce large amounts of sludge to be disposed with respect to the heterotrophic one.
Moreover, the risk of bacterial contamination is reduced, since the bacteria capable of surviving in the presence of hydrogen are few and generally consist of the same strains that are present in limited concentration in potable water.
The slowness of the process (generally, 1-2 kg of nitrates removed per m3 of supported biomass per day) is one of its main limitations.
In order to obviate the drawbacks of biological processes, a catalytic process has been proposed in which the nitrates are reduced with hydrogen, using catalysts based on palladium and copper on carriers having a controlled (bimodal) pore radius distribution or on activated carbon (U.S. Pat. No. 4,990,266).
The above indicated catalysts, however, tend to generate ammonia in an amount which exceeds the limits set by the applicable statutory provisions (0.5 mg/l): this occurs when working with amounts of hydrogen exceeding certain limits.
In order to obviate the above mentioned difficulties, catalysts are used which comprise palladium (which is capable of catalyzing the reduction of the nitrites, but not of the nitrates, to elementary nitrogen with high selectivity) mixed with catalyst which comprise palladium and copper, which are capable of catalyzing nitrate reduction (S. Horold et al, Catalysis Today, 17 (1993), 21-30).
The generation of ammonia in amounts exceeding the allowed limits, which occurs if the hydrogen concentration exceeds even low limits at which good nitrate removal activity occurs, causes the process to be of limited practical interest.
The aim of the present invention is to provide a water denitrification process which can reduce, with high specific activities, nitrates to elementary nitrogen, avoiding the generation of ammonia beyond the limits allowed by the applicable statutory provisions and/or of toxic substances of any other type.
An object is to provide a method as indicated above which can be performed continuously and at a convenient cost.
This aim, this object and others are unexpectedly achieved with the denitrification process according to the invention, which comprises a stage in which the water containing the nitrate ions to be eliminated is made to flow over an inert porous carrier which supports a transition metal capable of activating the molecular hydrogen, forming metallic hydrides, and on which denitrifying bacterial strains capable of surviving in anoxic conditions in the presence of hydrogen are made to adhere and grow.